1. Field of the Invention
The present invention relates to a method and apparatus for producing a toner used for developing an electrostatic image in, for example, copiers, electrostatic printing, printers, facsimiles and electrostatic recording.
2. Description of the Related Art
Developers used for developing an electrostatic image in, for example, electrophotography, electrostatic recording and electrostatic printing adhere to a latent electrostatic image bearing member on which an electrostatic image has been formed; then are transferred from the latent electrostatic image bearing member onto a recording medium (e.g., a recording paper sheet); and then are fixed on the surface of the recording medium. As have been known, such developers that develop an electrostatic image formed on the latent electrostatic image bearing member are roughly classified into two-component developers formed of a carrier and a toner and one-component developers requiring no carrier (magnetic or non-magnetic toners).
Conventionally, dry toners widely used in, for example, electrophotography, electrostatic recording and electrostatic printing have been so-called pulverized toners which are produced by finely pulverizing a melt-kneaded product of a toner binder (e.g., a styrene resin and a polyester resin), a colorant and the like.
Also, polymerization toners have recently been proposed, which are produced with the suspension polymerization method or the emulsion polymerization aggregation method.
However, the suspension polymerization method and the emulsion polymerization aggregation method pose a problem that the types of employable resins are limited.
In view of this, Japanese Patent Application Laid-Open (JP-A) No. 07-152202 or other literatures disclose a polymerization toner produced by a polymer dissolution suspension method involving volume shrinkage. In the polymer dissolution suspension method, toner materials are dispersed or dissolved in a volatile solvent such as an organic solvent having a low boiling point; and the resultant liquid is emulsified in an aqueous medium in the presence of a dispersing agent to form liquid droplets; and the volatile solvent is removed from the liquid droplets. Unlike the suspension polymerization method and the emulsion polymerization aggregation method, the polymer dissolution suspension method is advantageous in that a wider variety of resins can be used; in particular, a polyester resin can be used which is used for forming a full-color image having transparency and smoothness in image portions after fixing.
In this polymer dissolution suspension method, however, the dispersing agent must be used in the aqueous medium. Thus, the dispersing agent, which degrades chargeability of the formed toner particles, remains on their surfaces to impair environmental stability thereof. In order to avoid such an unfavorable phenomenon, the remaining dispersing agent must be removed using a very large amount of wash water, which is problematic.
In view of this, spray granulation methods have long been proposed as toner production methods using no aqueous medium (see, for example, JP-A No. 57-201248). The spray granulation methods produce particles through a process including: discharging a liquid containing toner materials melted or dissolved using various atomizers in the form of fine particles; and drying the fine particles to form particles. Thus, these spray granulation methods do not cause failures due to use of an aqueous medium.
However, the particles produced by conventional spray granulation methods are relatively coarse and large as well as broad in particle size distribution, problematically degrading the properties of the formed toner particles.
In view of this, there have been proposed a production method and a production apparatus for producing a toner through a process including: forming fine liquid droplets from nozzles utilizing piezoelectric pulsing; and drying and solidifying the fine liquid droplets to produce a toner (see, for example, Japanese Patent (JP-B) No. 3786034).
However, in the above toner production method and apparatus, nozzles correspond to piezoelectric elements on a one-on-one basis; i.e., the liquid droplets can be discharged from only one nozzle by one piezoelectric element. Thus, the number of liquid droplets discharged per unit of time is small to make their productivity low.
Also, there have been proposed a toner production method and apparatus for producing a toner through a process including: discharging a toner composition liquid from nozzles as fine liquid droplets to a solidification part by piezoelectric pulses converged with an acoustic lens; and drying and solidifying the fine liquid droplets (see, for example, JP-B No. 3786035).
However, also in the above toner production method and apparatus, the liquid droplets can be discharged from only one nozzle by one piezoelectric element. Thus, the number of liquid droplets discharged per unit of time is small to make their productivity low.
In view of this, there has been a toner production method including: expanding and contracting a piezoelectric element to vibrate a vibrating surface thereof facing a thin film containing a plurality of discharge holes (nozzles) thereby discharging liquid droplets of a toner composition fluid at a certain frequency; and solidifying the liquid droplets to form toner particles (see, for example, JP-A No. 2008-276146).
However, as in the above toner production method, when a plurality of discharge holes are provided per piezoelectric element, the time required that each discharge hole receives the vibration of the piezoelectric element varies with the distance from the discharge hole to the piezoelectric element. Thus, there arises time-lag between the liquid droplets discharged from the discharge holes, resulting in that the amounts of the liquid droplets discharged are different between the discharge holes.
In view of this, there have been proposed a method and apparatus for producing toner particles including: directly vibrating a thin film, which contains a plurality of discharge holes and is connected to a liquid chamber, by an electromechanical transducing unit disposed around the thin film to discharge a toner composition liquid as liquid droplets (film-vibrating discharge unit); and solidifying the liquid droplets to form toner particles (see, for example, JP-A No. 2008-281915). The above production method and apparatus for toner particles can directly vibrate the thin film containing the discharge holes and thus, can produce toner particles having a monodispersed particle size.